Low background counter



y 1961 s. D. SOFTKY 2,985,785

LOW BACKGROUND COUNTER Filed April 18, 1958 INVENTOR. SHELDON D. SOFTKY United States Patent LOW BACKGROUND COUNTER Sheldon D. Softky, Berkeley, Calif., assigu'or to Tracerlab, Inc., Waltham, Mass., a corporation of Massachusetts Filed Apr. 18, 1958, Ser. No. 729,340

4 Claims. (21. 313-93 The present invention relates in general to nuclear radiation detectors and more particularly to a beta counter of the gas fiow type which provides an extremely low background counting rate.

One of the most common uses of beta counters is for quantitative determination of the small amounts of radioisotope employed in tracer techniques and encountered in low level contamination studies. A practical limit on the minimum detectable amounts of these isotopes is established by the background count rate of the beta detector itself, where background count rate refers to the count rate observed in the counter when no radioactive sample is present. A portion of this background rate is due to cosmic radiation and to gamma radiation contributed by the contamination of materials in the general environment. The remaining portion of the rate may be considered as inherent in the counter. Its value will depend both on the radioactive contamination of the materials employed in the counter, and on the characteristics of these materials in regard to gamma ray radiation, photo-sensitivity and charge accumulation.

The present invention contemplates and has as a primary object the provision of an efilcient beta flow counter, operable with appropriate gas as either a proportional or Geiger-Mueller counter, and providing an extremely low background rate.

Other objects and advantages will now become apparent from the following description of a preferred embodiment of this invention wherein the single figure is a cross-sectional view of a beta counter, having generally circular symmetry, embodying the concepts of this invention.

With reference now to the drawing, the counter is seen to comprise an inverted cup shaped envelope 11, cast from a mixture of epoxy resin and petroleum derived lampblack, having a thin metallic coating 16, as for example, stainless steel, evaporated on a portion of its inner surface to provide a cathode. The lower end of this envelope is closed by a thin, opaque, electrically conducting gas tight membrane 14, for example, aluminum coated Mylar one mil thick, sealed to the envelope by a clamping ring 15 and in conductive contact with coating 16. A loop center electrode 13, such as three mil diameter stainless steel wire, is attached preferably by crimping to a conducting central support 12 which is preferably cast into the envelope 11. Cathode coating 16, as shown in the drawing, terminates without contacting central conducting support 12. Gas inlet and outlet tubes 17 and 18, respectively, are copper tubing also cast into the envelope 11.

While this embodiment indicates a particular electrode configuration and gas flow arrangement, it is to be understood that the principles of this invention may be applied to any of the conventional electrode arrangements. While the electrode arrangement illustrated is a conventional one, the unique low background characteristic is attributed to the envelope materials. Consideration of the requirements of this material indicates that it must pro- Patented May 23, 1961 vide mechanical rigidity, be readily capable of fabrica tion, yet provide minimum inherent background rate. The inherent background rate may be separated into several components as follows:

(1) Radioactivity in the counter materials;

(2) Counts resulting from photoelectric emission at the cathode initiated by both ultra-violet and visible light;

(3) Counts produced by gamma ray interaction with the counter materials; and

(4) Spurious counts resulting from accumulation of charge on interior insulation surfaces during periods of sample counting.

Use of an epoxy resin as the envelope material provides a ready means of fabrication in that it can be readily cast into a rigid form. The chemical history of epoxy resin eliminates radioactive contaminants found in metals, glass and cellulose compounds; and the thinness of the evaporated metal coating minimizes any contribution from the cathode. Admixing lampblack derived from petroleum or other fossil sources (thus being uncontaminated with radioactivity), with this resin does not adversely affect its casting properties, yet it renders the envelope opaque to ultra-violet and visible light, and further, it renders the insulating area of the inner surface in the region of the center electrode support 12 slightly conducting, hence preventing charge accumulation. This decreased resistivity, which allows the charge to drain off, therefore greatly reduces this memory eifect.

The low atomic number of the constituents of this envelope considerably reduces the probability of gamma ray interactions over that present in metals and hence minimizes that background contribution.

Counters embodying the principles of this invention have been constructed using Epon plastic which contains substantially no carbon-14 (Shell Chemical Co.) with approximately eight milligrams to one gram of petroleum derived lampblack added to each gram of Epon resin, and operated using commercial Geiger gas such as that described in the Whitman Patent 2,712,088. For counters which are to withstand a 5000 volt anode potential, the preferred proportion is 8 milligrams of lampblack per gram of resin. The counting characteristics were satisfactory (two hundred volt plateau with a slope of two percent per one hundred volts) and the background rate after subtraction of cosmic rays (by conventional means, for example, surrounding the counter with a ring of other Geiger counters and a massive iron shield) was less than one count per minute for a counter with a window opening of 1.3 inch diameter. By using methane gas, the device functions as a proportional counter.

In view of the fact that numerous modifications may now be made by those skilled in the art, the invention herein is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. Apparatus for detecting beta radiation comprising, a generally cup shaped envelope formed from a mixture of epoxy resin and fossil derived lampblack, a central electrode extending into said envelope, a thin conductive film electrode formed on the inner surface of said envelope except in the region of said central electrode, counter gas inlet and outlet means, and a gas tight thin membrane sealing the open end of said envelope.

2. Apparatus for detecting beta radiation comprising, a generally cup shaped envelope formed from a mixture of epoxy resin and petroleum derived lampblack, said lampblack being efiective to render said envelope substantially opaque and slightly conducting, a central electrode, a conductive support for said central electrode extending through said envelope, a relatively thin conductive evaporated film electrode adhered to the inner surface of said envelope except in the region of said central electrode support, conductive membrane for sealing the open end of said envelope and conductively coupled to said film electrode, and means for flowing a counting gas through said envelope.

3. Apparatus for detecting beta radiation inaccordance with claim 2 wherein said envelope is formed of a mixture of eight milligrams to one gram of lampblack for each gram of epoxy resin.

4. Apparatus for detecting beta radiation with relatively low background counting rate comprising, a generally inverted cup-shaped envelope cast from a mixture of epoxy resin and petroleum derived lampblack in the proportion of eight milligrams to one gram of lampblack for each gram of resin, said envelope being thereby opaque to visible and ultraviolet light while slightly conductive and substantially uncontaminated with radioactivity, a conductive electrode support extending through the upper portion of said envelope, a central electrode disposed on the inner end of said support substantially centrally within said cup-shaped envelope, a relatively thin evaporated film electrode adhered to the inner surface of said cupshaped electrode and extending from a region spaced from said central electrode to the lower edge thereof, a clamping ring secured to said lower edge, a conductive opaque thin membrane secured to said lower edge by said clamping ring, said membrane being in conductive contact with said film electrode, and inlet and outlet tubes formed in said envelope to permit the fiow of counting gas therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 2,596,080 Raper et a1. May 6, 1952 2,605,435 Krasnow July 29, 1952 2,742,586 Friedman Apr. 17, 1956 

